Title: HST Imaging of Nearby Active Galaxies: The Trouble with the "Unified Scheme"
Speaker: Matt MAlkan, UCLA
ABSTRACT
It is tempting to suppose that the two types of active galactic nuclei--
Seyfert 1's and Seyfert 2's are really the same kind of object. The most
popular "Unified Scheme" claims that in both cases the same intrinsic central
engine is surrounded by a thick torus, and that the appearance depends only on
the viewing angle, with the Seyfert 1's viewed closer to pole-on, and the
Seyfert 2's viewed closer to edge-on. We have obtained deep optical Planetary
Camera images of over 200 nearby Seyfert galaxies which contradict this
hypothesis. The images reveal that the Seyfert 2 nuclei reside in galaxies
which have systematically dustier, later-type central morphologies, than do
the Seyfert 1's. We observe that the view to a Seyfert 2 is more likely to be
obstructed by galactic dust lanes on scales of 100 or more parsecs, which are
sufficiently optically thick to obscure our view of the inner Seyfert 1
characteristics. Since HST now allows us to see much of the galactic
obscuration in the Seyfert 2 galaxies, there is now little motivation to
believe in the existence of a thick torus on a much smaller size scale. We
review other observations and conclude that basic ideas of the Unified Scheme,
such as a systematic difference in orientation between the central engines in
Seyfert 1's and 2's, are probably wrong.